Control of irrigation water flow to plants in agriculture and horticulture is essential to minimize wastage of the resource and salination, as well as to ensure that plants receive sufficient water for productive growth. Overwatering may also be disadvantageous to water sensitive plants. In general, irrigation systems currently in use rely on direct monitoring by observation to determine watering times. Where automation is essential this has generally been achieved by the use of timers.
Watering by observation has the disadvantage that soil moisture conditions below the visible soil surface may not be readily apparent, which may lead to over or under watering. Timers also are acting without regard for the status of the soil at the time of watering. Whilst the watering regime may be modified in each case by having regard to rainfall, and in the case of timer apparatus it has been proposed to interface the output of a rain sensor to modify the timed irrigation regime, the result remains independent of soil condition.
It has been proposed to utilize sensors to monitor soil moisture conditions, such sensors generally being of the conductivity type working on the basis of an electrical current monitoring the amount of moisture in the soil. However, these have proved to be less than reliable in service and additionally must be calibrated for each soil type to give a signal indicative of a selected moisture status. Derivative technology includes ceramic block and ceramic probe sensors, which substantially overcome the reliability problems associated with conductivity type sensors. However, the sensors are extremely expensive and still suffer from calibration problems where different soil types exist.